(MERAF) for the Base Metals Smelting Sector - CCME

3.2.10. Noranda Inc., Division CEZ, Valleyfield, Québec 89
Figure 18 contains a schematic block flow diagram for the Zinc Plant. This
shows the general sequence of the major unit operations, from the receipt of raw
materials to the production of metals. When these unit operations are referred to
in the text, the corresponding words starts with an upper case letter. The
diagram is highly simplified and is included solely to aid in the discussion and
understanding of the pollution control at the facility.
Zinc concentrates are first processed in fluidized bed roasters where sulphide
sulphur is oxidized to sulphur dioxide and zinc sulphide minerals to zinc oxides.
The iron sulphide minerals are converted to iron oxide and zinc ferrite.
Roaster off-gases are treated in turn by a waste heat boiler, electrostatic
precipitator and venturi scrubber before being directed to a sulphuric acid plant
for recovery of sulphur dioxide.
The scrubber produces a weak acid which is treated to remove selenium and
mercury prior to being neutralized with lime to precipitate gypsum and metal
hydroxides and transferred to an on -site area for impoundment as a mining
residue.
The zinc calcine from the roaster is leached with sulphuric acid in three stages to
extract zinc and other metals. In the first two stages, zinc oxides are selectively
leached while zinc ferrites are not. In the third stage, the leach residue is
subjected to higher temperatures and free acid concentrations to extract zinc
from the more refractory zinc ferrites, resulting in dissolution of iron as well as
zinc. Iron is removed from solution with ammonia as ammonium jarosite, which is
stabilized, and then impounded on-site.
The leach liquor contains zinc sulphate and impurities that must be removed
before zinc can be electrowon from solution. Solution purification is achieved
primarily by adding zinc dust to cement out impurities. Minor amounts of
antimony trioxide and arsenic trioxide are also used to control cobalt levels. The
resulting purification residue is further processed to produce copper cake and
metallic cadmium for sale.
Zinc is then recovered by electrowinning from the purified zinc sulphate solution,
and the spent electrolyte is reused in the leaching section.
Zinc cathodes are mechanically stripped, melted and cast into slabs for shipment
to customers. Some molten zinc is atomized to produce zinc dust for use in
solution purification.
89 Hatch Associates, Review of Environmental Releases for the Base Metals Smelting Sector,
prepared for Environment Canada, dated November 2000
84